This invention relates to semiconductor processing and, more particularly, to a novel method for forming buried contacts in MOS devices.
In the past, the prior art utilized metal interconnects and contacts to connect doped semiconductor regions or to connect to the gate of an MOS integrated circuit. However, as the density of the integrated circuit devices on a given size chip increased, it was found that there could be a significant saving in space by eliminating the space consuming metal contacts and interconnects. Accordingly, the use of polycrystalline silicon interconnects and buried contacts became a viable solution for the space problem and the use of polycrystalline silicon interconnects is now widespread. However, this widespread use of polycrystalline silicon has brought about certain problems when these techniques were applied, for example, to a silicon-on-insulator device which device, by its very nature, has a thin silicon epitaxial layer deposited on an insulating substrate.
A "buried contact" may be defined as a contact scheme which provides a low resistance, direct, ohmic contact between two polycrystalline silicon (polysilicon) layers or between a polysilicon layer and either bulk silicon or a layer of epitaxially grown silicon with no appreciable penalty for either the misalignment or any undesirably formed junction at the point of connection. Despite the advantages of the use of polysilicon interconnects and buried contacts, there have, nevertheless, been certain technical problems which result from the fact that the silicon layer in an SOS device is only about 6,000 angstroms in thickness. In a recently issued patent to A.G.F. Dingwall, entitled "BURIED CONTACT CONFIGURATION FOR CMOS/SOS INTEGRATED CIRCUITS," U.S. Pat. No. 4,196,443 which issued on Apr. 1, 1980 and assigned to the same assignee as the subject application, there is described various buried contact opening configurations formed in the insulating layer overlying the layer of semiconductor material and through which the buried contact is made. The shape of the opening for the buried contact is designed to preclude the complete removal of the epitaxial silicon material in the event of a misalignment. In the event of a misalignment, the described opening will insure that there is sufficient contact and silicon to form a useful device. However, the above-mentioned patent does not eliminate the possibility of the inadvertent removal of portions of the epitaxial silicon body.